Continuous measurement of alveolo-arterial gradients at ambient and anoxic levels

¹ Division of Cardiology, Department of Medicine, University of Southern California School of Medicine; and Los Angeles County Hospital, Los Angeles, California

In 18 normal subjects continuous alveolo-arterial differences were measured at decreasing levels of inspired pO₂ from ambient to 12% oxygen. A mean difference of 12.1 mm Hg decreased gradually to virtual obliteration of the gradient at low levels of inspired pO₂. A modified Clark electrode lying within a Riley needle continuously recorded arterial O₂ tensions and, together with a recording alveolar O₂ electrode, inscribed a continuous visual alveolo-arterial difference. The response of arterial pO₂ to an anoxic level, though maximal in the 1st min (74% of the total change in 4 min), continued to change at the rate of 2 %/min during and after the 4th min. Continuously changing levels of ventilation and arterial pO₂ and pCO₂, as described, make it unlikely that the steady state is ever truly present during acute exposure to anoxia.